Heat exchanger



FIG. 1 is a perspective view of a heat exchanger, showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a top view thereof;

FIG. 4 is a right side elevational view thereof;

FIG. 5 is a left side elevational view thereof;

FIG. 6 is a back elevational view thereof; and,

FIG. 7 is a bottom view thereof.

The ornamental design for a heat exchanger, as shown and described.